fix(tui): undo latest pending prompt

[kitlangton]

What

Make /undo remove the newest pending user prompt before reverting projected session history. This applies equally to queued and steering follow-ups and restores the removed prompt to the composer.

Fixes #39736.

Before / After

Before

1. A queued or steering follow-up was admitted and shown optimistically while a response was active.
2. /undo selected that user message but sent it to the history revert endpoint.
3. Because the input had not been promoted into history, revert failed with MessageNotFoundError and the pending prompt remained.

After

1. /undo selects the newest pending user input by admission order.
2. It cancels that input through the existing durable pending-input endpoint and restores its content to the composer.
3. If promotion wins the cancellation race, the expected conflict falls back to normal history revert.
4. With no pending user input, /undo retains its existing projected-history behavior.

How

• packages/tui/src/routes/session/index.tsx prioritizes pendingUsers() when selecting the undo target.
• packages/tui/src/routes/session/undo.ts routes pending targets to cancellation and handles only the expected promotion conflict as a revert fallback.
• packages/tui/test/cli/tui/undo.test.ts covers projected, pending, promotion-race, and transport-failure paths.

Scope

• Uses the existing session.pending.cancel API and session.input.cancelled event; no protocol or core lifecycle changes.
• Synthetic pending inputs and compaction barriers remain untouched.
• Redo remains specific to staged history reverts; cancelling a pending prompt does not create a redo boundary.

Testing

• packages/tui: bun run test, 612 passed and 5 skipped.
• packages/tui: bun typecheck.
• Push hook: repository Turbo typecheck, 33 packages passed.
• OpenCode Drive scripted TUI walkthrough with a simulated model, covering queued cancellation, steering cancellation, and projected-history revert.

Demo

The recording uses OpenCode Drive with a simulated streaming model. The first two /undo actions remove and restore a steering prompt, then a queued prompt. After streaming completes, the third /undo follows the existing history-revert path and displays 1 message reverted.

recording-89a775d2-ff93-4bb1-8746-a66298ebc869.mp4

Flow

flowchart TD
    Undo[/undo/] --> Pending{Pending user input?}
    Pending -->|Yes| Cancel[Cancel newest pending input]
    Cancel --> Conflict{Already promoted?}
    Conflict -->|No| Restore[Restore prompt to composer]
    Conflict -->|Yes| Revert[Stage history revert]
    Pending -->|No| Revert
    Revert --> Restore

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[github-actions]

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